The present invention relates to an oil condition sensor used to check the degree of contamination of automotive transmission oil or engine oil.
Such an oil condition sensor is mounted in an oil container for automotive transmission oil or engine oil and is used to check how the oil is contaminated with iron powder or other metallic powder that are produced when automotive parts are abraded against each other. A typical such sensor is disclosed in JP patent publication 2002-286697 (see especially its FIGS. 1–4). It includes a rod adapted to be immersed in oil. The rod carries magnets at its tip. A pair of electrodes are provided around the magnets, axially spaced from and opposed to each other. The sensor produces a signal corresponding to the resistance in the oil between the electrodes, which in turn corresponds to the amount of metallic powder in the oil.
At least one of the electrodes is a resistor. Any conductive material in the oil such as iron powder tends to be magnetically attracted toward the outer peripheries of the magnets and stuck on an end face of the resistor. The resistance between the electrodes is a function of the area of a conductive material covering the end face of the resistor. Specifically, the resistance Ω between the electrodes is given by R0×(100/S), where R0 is the specific resistance of the resistor, and S is the percentage of the area of the conductive material covering the end face of the resistor. From this equation, one can see that the higher the value S, i.e. the degree of contamination of the oil, the smaller the resistance between the electrodes. The resistor forming one of the electrodes may be made of a conductive resin containing carbon, a good conductor, or a conductive ceramic material. According to this publication, instead of using a resistor for at least one of the electrodes, a resistive material may be applied to the outer surface of a tubular insulating cover that is disposed between magnets and the electrodes.
As explained above, the resistor or the resistive material is an essential element for the oil condition sensor of this publication. Such a resistor or resistive material tends to be internally stressed when the load and/or temperature changes. Such internal stresses in turn cause fluctuations in the specific resistance R0 of the resistor or resistive material. Because the specific resistance R0 easily fluctuates, it is impossible to stably and reliably detect the resistance between the electrodes, so that no accurate determination of the degree of contamination of oil is possible.
An object of the invention is to provide an oil condition sensor which can accurately determine the degree to which oil is contaminated with metallic powder.